Conventionally, in radiotherapy which administers treatment by irradiating an affected area of a patient with therapeutic radiation, it is important to position the therapeutic radiation with high precision and accurately measure the position of the affected area to be irradiated. This is because these operations allow a margin of a region irradiated with the therapeutic radiation to be reduced, thereby reducing a radiation exposure dose of normal tissue existing around the affected area.
However, when the affected area moves along with breathing, it is necessary to increase the margin of the irradiated region by taking the respiratory movements into consideration. To deal with this, breathing synchronized radiotherapy is used which irradiates an affected area of the patient with therapeutic radiation only in a particular breathing phase in order to reduce the margin of the irradiated region resulting from the respiratory movements and thereby reduce the radiation exposure dose of normal tissue. The breathing synchronized radiotherapy, which eliminates the need to give much consideration to the respiratory movements of an affected area, allows the margin of the irradiated region to be reduced.
In the breathing synchronized radiotherapy described above, markers are used to determine the breathing phase. Known examples of the markers include an in vitro marker which is kept track of via a pressure sensor pasted to body surface of the patient or an inhaled air sensor coupled to the mouth of the patient and an in vivo marker used to keep track of the marker inserted into the body, a bone structure, and diaphragmatic breathing exercise by continuous X-ray fluoroscopy (Patent Literatures 1 and 2).